Most alternators have the rotating field and the stationary armature.The rotating-field type alternator has several advantages over the rotating-armature type alternator.
(1)A stationary armature is more easily insulated for the high voltage for which the alternator is designed.This generated voltage may be as high as 33 kV.
(2)The armature windings can be braced better mechanically against high electromagnetic forces due to large short-circuit currents when the armature windings are in the stator.
(3)The armature windings, being stationary,are not subjected to vibration and centrifugal forces.
(4)The output current can be taken directly from fixed terminals on the stationary armature without using slip rings,brushes,etc.
(5)The rotating field is supplied with direct current.Usually the field voltage is between 100 to 500 volts.Only two slip rings are required to provide direct current for the rotating field,while at least three slip rings would be required for a rotating armature.The insulation of the relatively low voltage slip rings from the shaft can be provided easily.
(6)The bulk and weight of the armature winding are substantially greater than the windings of the field poles.The size of the machine is,therefore,reduced.
(7)Rotating field is comparatively light and can be constructed for high speed rotation.The armatures of large alternators are forced cooled with circulating gas or liquids.
(8)The stationary armature may be cooled more easily because the armature can be made large to provide a number of cooling ducts.
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